node: manually implement Debug

I got too irritated today with the default Debug implementation of
hg::revlog::Node while playing with a new parser. This isn't quite
what I wanted, but it wasn't much code and it at least gives you
output that's easy to visually compare to a node.hex()ed identifier
from the Python side of things.

Sadly, this doesn't influence the output in lldb or the VSCode
debugger extension that uses lldb under the covers, but it at least
means debug prints are a little more useful.

Differential Revision: https://phab.mercurial-scm.org/D12608

rust/hg-core/src/revlog/node.rs

@@ -1,421 +1,430 b''
 // Copyright 2019-2020 Georges Racinet <[e-mail]>
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 
 //! Definitions and utilities for Revision nodes
 //!
 //! In Mercurial code base, it is customary to call "a node" the binary SHA
 //! of a revision.
 
 use crate::errors::HgError;
 use bytes_cast::BytesCast;
 use std::convert::{TryFrom, TryInto};
 use std::fmt;
 
 /// The length in bytes of a `Node`
 ///
 /// This constant is meant to ease refactors of this module, and
 /// are private so that calling code does not expect all nodes have
 /// the same size, should we support several formats concurrently in
 /// the future.
 pub const NODE_BYTES_LENGTH: usize = 20;
 
 /// Id of the null node.
 ///
 /// Used to indicate the absence of node.
 pub const NULL_NODE_ID: [u8; NODE_BYTES_LENGTH] = [0u8; NODE_BYTES_LENGTH];
 
 /// The length in bytes of a `Node`
 ///
 /// see also `NODES_BYTES_LENGTH` about it being private.
 const NODE_NYBBLES_LENGTH: usize = 2 * NODE_BYTES_LENGTH;
 
 /// Default for UI presentation
 const SHORT_PREFIX_DEFAULT_NYBBLES_LENGTH: u8 = 12;
 
 /// Private alias for readability and to ease future change
 type NodeData = [u8; NODE_BYTES_LENGTH];
 
 /// Binary revision SHA
 ///
 /// ## Future changes of hash size
 ///
 /// To accomodate future changes of hash size, Rust callers
 /// should use the conversion methods at the boundaries (FFI, actual
 /// computation of hashes and I/O) only, and only if required.
 ///
 /// All other callers outside of unit tests should just handle `Node` values
 /// and never make any assumption on the actual length, using [`nybbles_len`]
 /// if they need a loop boundary.
 ///
 /// All methods that create a `Node` either take a type that enforces
 /// the size or return an error at runtime.
 ///
 /// [`nybbles_len`]: #method.nybbles_len
-#[derive(Copy, Clone, Debug, PartialEq, BytesCast, derive_more::From)]
+#[derive(Copy, Clone, PartialEq, BytesCast, derive_more::From)]
 #[repr(transparent)]
 pub struct Node {
     data: NodeData,
 }
 
+impl fmt::Debug for Node {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let n = format!("{:x?}", self.data);
+        // We're using debug_tuple because it makes the output a little
+        // more compact without losing data.
+        f.debug_tuple("Node").field(&n).finish()
+    }
+}
+
 /// The node value for NULL_REVISION
 pub const NULL_NODE: Node = Node {
     data: [0; NODE_BYTES_LENGTH],
 };
 
 /// Return an error if the slice has an unexpected length
 impl<'a> TryFrom<&'a [u8]> for &'a Node {
     type Error = ();
 
     #[inline]
     fn try_from(bytes: &'a [u8]) -> Result<Self, Self::Error> {
         match Node::from_bytes(bytes) {
             Ok((node, rest)) if rest.is_empty() => Ok(node),
             _ => Err(()),
         }
     }
 }
 
 /// Return an error if the slice has an unexpected length
 impl TryFrom<&'_ [u8]> for Node {
     type Error = std::array::TryFromSliceError;
 
     #[inline]
     fn try_from(bytes: &'_ [u8]) -> Result<Self, Self::Error> {
         let data = bytes.try_into()?;
         Ok(Self { data })
     }
 }
 
 impl From<&'_ NodeData> for Node {
     #[inline]
     fn from(data: &'_ NodeData) -> Self {
         Self { data: *data }
     }
 }
 
 impl fmt::LowerHex for Node {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         for &byte in &self.data {
             write!(f, "{:02x}", byte)?
         }
         Ok(())
     }
 }
 
 #[derive(Debug)]
 pub struct FromHexError;
 
 /// Low level utility function, also for prefixes
 fn get_nybble(s: &[u8], i: usize) -> u8 {
     if i % 2 == 0 {
         s[i / 2] >> 4
     } else {
         s[i / 2] & 0x0f
     }
 }
 
 impl Node {
     /// Retrieve the `i`th half-byte of the binary data.
     ///
     /// This is also the `i`th hexadecimal digit in numeric form,
     /// also called a [nybble](https://en.wikipedia.org/wiki/Nibble).
     pub fn get_nybble(&self, i: usize) -> u8 {
         get_nybble(&self.data, i)
     }
 
     /// Length of the data, in nybbles
     pub fn nybbles_len(&self) -> usize {
         // public exposure as an instance method only, so that we can
         // easily support several sizes of hashes if needed in the future.
         NODE_NYBBLES_LENGTH
     }
 
     /// Convert from hexadecimal string representation
     ///
     /// Exact length is required.
     ///
     /// To be used in FFI and I/O only, in order to facilitate future
     /// changes of hash format.
     pub fn from_hex(hex: impl AsRef<[u8]>) -> Result<Node, FromHexError> {
         let prefix = NodePrefix::from_hex(hex)?;
         if prefix.nybbles_len() == NODE_NYBBLES_LENGTH {
             Ok(Self { data: prefix.data })
         } else {
             Err(FromHexError)
         }
     }
 
     /// `from_hex`, but for input from an internal file of the repository such
     /// as a changelog or manifest entry.
     ///
     /// An error is treated as repository corruption.
     pub fn from_hex_for_repo(hex: impl AsRef<[u8]>) -> Result<Node, HgError> {
         Self::from_hex(hex.as_ref()).map_err(|FromHexError| {
             HgError::CorruptedRepository(format!(
                 "Expected a full hexadecimal node ID, found {}",
                 String::from_utf8_lossy(hex.as_ref())
             ))
         })
     }
 
     /// Provide access to binary data
     ///
     /// This is needed by FFI layers, for instance to return expected
     /// binary values to Python.
     pub fn as_bytes(&self) -> &[u8] {
         &self.data
     }
 
     pub fn short(&self) -> NodePrefix {
         NodePrefix {
             nybbles_len: SHORT_PREFIX_DEFAULT_NYBBLES_LENGTH,
             data: self.data,
         }
     }
 
     pub fn pad_to_256_bits(&self) -> [u8; 32] {
         let mut bits = [0; 32];
         bits[..NODE_BYTES_LENGTH].copy_from_slice(&self.data);
         bits
     }
 }
 
 /// The beginning of a binary revision SHA.
 ///
 /// Since it can potentially come from an hexadecimal representation with
 /// odd length, it needs to carry around whether the last 4 bits are relevant
 /// or not.
 #[derive(Debug, PartialEq, Copy, Clone)]
 pub struct NodePrefix {
     /// In `1..=NODE_NYBBLES_LENGTH`
     nybbles_len: u8,
     /// The first `4 * length_in_nybbles` bits are used (considering bits
     /// within a bytes in big-endian: most significant first), the rest
     /// are zero.
     data: NodeData,
 }
 
 impl NodePrefix {
     /// Convert from hexadecimal string representation
     ///
     /// Similarly to `hex::decode`, can be used with Unicode string types
     /// (`String`, `&str`) as well as bytes.
     ///
     /// To be used in FFI and I/O only, in order to facilitate future
     /// changes of hash format.
     pub fn from_hex(hex: impl AsRef<[u8]>) -> Result<Self, FromHexError> {
         let hex = hex.as_ref();
         let len = hex.len();
         if len > NODE_NYBBLES_LENGTH || len == 0 {
             return Err(FromHexError);
         }
 
         let mut data = [0; NODE_BYTES_LENGTH];
         let mut nybbles_len = 0;
         for &ascii_byte in hex {
             let nybble = match char::from(ascii_byte).to_digit(16) {
                 Some(digit) => digit as u8,
                 None => return Err(FromHexError),
             };
             // Fill in the upper half of a byte first, then the lower half.
             let shift = if nybbles_len % 2 == 0 { 4 } else { 0 };
             data[nybbles_len as usize / 2] |= nybble << shift;
             nybbles_len += 1;
         }
         Ok(Self { data, nybbles_len })
     }
 
     pub fn nybbles_len(&self) -> usize {
         self.nybbles_len as _
     }
 
     pub fn is_prefix_of(&self, node: &Node) -> bool {
         let full_bytes = self.nybbles_len() / 2;
         if self.data[..full_bytes] != node.data[..full_bytes] {
             return false;
         }
         if self.nybbles_len() % 2 == 0 {
             return true;
         }
         let last = self.nybbles_len() - 1;
         self.get_nybble(last) == node.get_nybble(last)
     }
 
     /// Retrieve the `i`th half-byte from the prefix.
     ///
     /// This is also the `i`th hexadecimal digit in numeric form,
     /// also called a [nybble](https://en.wikipedia.org/wiki/Nibble).
     pub fn get_nybble(&self, i: usize) -> u8 {
         assert!(i < self.nybbles_len());
         get_nybble(&self.data, i)
     }
 
     fn iter_nybbles(&self) -> impl Iterator<Item = u8> + '_ {
         (0..self.nybbles_len()).map(move |i| get_nybble(&self.data, i))
     }
 
     /// Return the index first nybble that's different from `node`
     ///
     /// If the return value is `None` that means that `self` is
     /// a prefix of `node`, but the current method is a bit slower
     /// than `is_prefix_of`.
     ///
     /// Returned index is as in `get_nybble`, i.e., starting at 0.
     pub fn first_different_nybble(&self, node: &Node) -> Option<usize> {
         self.iter_nybbles()
             .zip(NodePrefix::from(*node).iter_nybbles())
             .position(|(a, b)| a != b)
     }
 }
 
 impl fmt::LowerHex for NodePrefix {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let full_bytes = self.nybbles_len() / 2;
         for &byte in &self.data[..full_bytes] {
             write!(f, "{:02x}", byte)?
         }
         if self.nybbles_len() % 2 == 1 {
             let last = self.nybbles_len() - 1;
             write!(f, "{:x}", self.get_nybble(last))?
         }
         Ok(())
     }
 }
 
 /// A shortcut for full `Node` references
 impl From<&'_ Node> for NodePrefix {
     fn from(node: &'_ Node) -> Self {
         NodePrefix {
             nybbles_len: node.nybbles_len() as _,
             data: node.data,
         }
     }
 }
 
 /// A shortcut for full `Node` references
 impl From<Node> for NodePrefix {
     fn from(node: Node) -> Self {
         NodePrefix {
             nybbles_len: node.nybbles_len() as _,
             data: node.data,
         }
     }
 }
 
 impl PartialEq<Node> for NodePrefix {
     fn eq(&self, other: &Node) -> bool {
         Self::from(*other) == *self
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
     const SAMPLE_NODE_HEX: &str = "0123456789abcdeffedcba9876543210deadbeef";
     const SAMPLE_NODE: Node = Node {
         data: [
             0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef, 0xfe, 0xdc, 0xba,
             0x98, 0x76, 0x54, 0x32, 0x10, 0xde, 0xad, 0xbe, 0xef,
         ],
     };
 
     /// Pad an hexadecimal string to reach `NODE_NYBBLES_LENGTH`
     /// The padding is made with zeros.
     pub fn hex_pad_right(hex: &str) -> String {
         let mut res = hex.to_string();
         while res.len() < NODE_NYBBLES_LENGTH {
             res.push('0');
         }
         res
     }
 
     #[test]
     fn test_node_from_hex() {
         let not_hex = "012... oops";
         let too_short = "0123";
         let too_long = format!("{}0", SAMPLE_NODE_HEX);
         assert_eq!(Node::from_hex(SAMPLE_NODE_HEX).unwrap(), SAMPLE_NODE);
         assert!(Node::from_hex(not_hex).is_err());
         assert!(Node::from_hex(too_short).is_err());
         assert!(Node::from_hex(&too_long).is_err());
     }
 
     #[test]
     fn test_node_encode_hex() {
         assert_eq!(format!("{:x}", SAMPLE_NODE), SAMPLE_NODE_HEX);
     }
 
     #[test]
     fn test_prefix_from_to_hex() -> Result<(), FromHexError> {
         assert_eq!(format!("{:x}", NodePrefix::from_hex("0e1")?), "0e1");
         assert_eq!(format!("{:x}", NodePrefix::from_hex("0e1a")?), "0e1a");
         assert_eq!(
             format!("{:x}", NodePrefix::from_hex(SAMPLE_NODE_HEX)?),
             SAMPLE_NODE_HEX
         );
         Ok(())
     }
 
     #[test]
     fn test_prefix_from_hex_errors() {
         assert!(NodePrefix::from_hex("testgr").is_err());
         let mut long = format!("{:x}", NULL_NODE);
         long.push('c');
         assert!(NodePrefix::from_hex(&long).is_err())
     }
 
     #[test]
     fn test_is_prefix_of() -> Result<(), FromHexError> {
         let mut node_data = [0; NODE_BYTES_LENGTH];
         node_data[0] = 0x12;
         node_data[1] = 0xca;
         let node = Node::from(node_data);
         assert!(NodePrefix::from_hex("12")?.is_prefix_of(&node));
         assert!(!NodePrefix::from_hex("1a")?.is_prefix_of(&node));
         assert!(NodePrefix::from_hex("12c")?.is_prefix_of(&node));
         assert!(!NodePrefix::from_hex("12d")?.is_prefix_of(&node));
         Ok(())
     }
 
     #[test]
     fn test_get_nybble() -> Result<(), FromHexError> {
         let prefix = NodePrefix::from_hex("dead6789cafe")?;
         assert_eq!(prefix.get_nybble(0), 13);
         assert_eq!(prefix.get_nybble(7), 9);
         Ok(())
     }
 
     #[test]
     fn test_first_different_nybble_even_prefix() {
         let prefix = NodePrefix::from_hex("12ca").unwrap();
         let mut node = Node::from([0; NODE_BYTES_LENGTH]);
         assert_eq!(prefix.first_different_nybble(&node), Some(0));
         node.data[0] = 0x13;
         assert_eq!(prefix.first_different_nybble(&node), Some(1));
         node.data[0] = 0x12;
         assert_eq!(prefix.first_different_nybble(&node), Some(2));
         node.data[1] = 0xca;
         // now it is a prefix
         assert_eq!(prefix.first_different_nybble(&node), None);
     }
 
     #[test]
     fn test_first_different_nybble_odd_prefix() {
         let prefix = NodePrefix::from_hex("12c").unwrap();
         let mut node = Node::from([0; NODE_BYTES_LENGTH]);
         assert_eq!(prefix.first_different_nybble(&node), Some(0));
         node.data[0] = 0x13;
         assert_eq!(prefix.first_different_nybble(&node), Some(1));
         node.data[0] = 0x12;
         assert_eq!(prefix.first_different_nybble(&node), Some(2));
         node.data[1] = 0xca;
         // now it is a prefix
         assert_eq!(prefix.first_different_nybble(&node), None);
     }
 }
 
 #[cfg(test)]
 pub use tests::hex_pad_right;